There is a conventional electron tube wherein, for example, an insulating substrate holding electrodes is housed in a housing. In the electron tube of this kind, if electrons are incident into the insulating substrate holding the electrodes to electrify the substrate, it may result in degradation of withstand voltage characteristics between the electrodes. For improving the withstand voltage characteristics, for example in the case of the photomultiplier described in Patent Literature 1, chrome oxide is applied onto the surface of the ceramic substrate and then fired to form a chrome oxide film.